On noon Friday, three days before the storm hit NO, the forecast track from the National Hurricane Center still was forecasting a track that would have Katrina hitting Pensacola, FL and the panhandle area. NO was on the far edge of the expected track. No one expected NO to get hit at that point.
Look at the far right side of the white circle. That's the zone of uncertainty for the forecast.
NOLA is right on the edge of it - and the track shifted by the 5pm advisory and it was running right over NOLA.
My parents live along the Texas Gulf Coast. During Rita's approach, their town ordered a mandatory evacuation 60 hours before any anticipated landfall, even though the forecast track was 100 miles to the north - because the town was in that zone of uncertainty. Rita ended up missing them by a wide margin - but if you wait for the track to become absolutely certain, it's too late.
Nagin should have ordered evacuations Friday night when two straight advisories (5pm, 11pm) had NOLA dead center in the path. Instead, he spent Saturday asking for legal advice about issuing a mandatory evacuation. Don't you think he should have worked that out, say, LAST YEAR?!?
11 AM called for Pensacola. 5 PM the models all pointed to N.O. but the NHC played it safe and did not move the forecast track further than MS. Then at 11 PM (10 CDT), they moved it to N.O. proper. It was all in the discussions that they put out.